1. Field of the Invention
The present invention relates to an improvement in a solid-state image sensor suitable to realize a compact type image pickup camera.
2. Description of the Prior Art
Recently, a signal reading system using a solid-state image sensor utilizing a CCD device, MOS device or CPD device has come into practical use. Further, a pile up type image sensor with a photoconductive film piled thereon has been developed for the respective above-mentioned signal reading methods.
But in each signal reading system, there is too much blooming. That is, excessively excited signal carriers are partially generated in an image pickup area where a spot light of high intensity is irradiated, and the signal carriers which generate blooming are diffused to an adjacent area, because the dynamic-range for incident light is narrow in each of the above-mentioned signal reading systems.
The prior art is elucidated with reference to the drawings FIG. 1 to FIG. 5, wherein FIG. 4 and FIG. 5 are response graphs for general prior art devices.
FIG. 1 is a plan view of a conventional solid-state color image sensor with a color filter thereon for general use.
FIG. 2 is a sectional view taken on line X--X' of the solid-state color image sensor of FIG. 1 when a spot light is irradiated thereon.
As shown in FIG. 1 and FIG. 2, for example, the conventional solid-state color image sensor comprises a picture element area 2 disposed on the center of a P-type semiconductor substrate 1. The picture element area 2 is formed by including photo sensors 12, such as photo diodes, for sensing an optical image of incident light .circle.A irradiated thereon, and a device for transferring an optical signal, such as charge coupled device. A driving circuit area 3 is formed at the circumference of the picture element area 2, so as to drive the picture element area 2, and an insulation film 4 is disposed on the driving circuit area 3. A wiring conductor 6 and an electrode 7 for connecting a transparent electrode 9 are disposed on the insulation film 4. The wiring conductor 6 is connected to a terminal (not shown) of the driving circuit area 3 through an opening (not shown) of the insulation film 4 and extended to a pad 5 for external wiring. The photoconductive film 8 is, for example, a ZnSe--Zn.sub.1-x Cd.sub.x Te or an amorphous silicon. A transparent electrode 9 is superposingly formed on the picture elements area 2. Further a color filter 11 is bonded or formed above the picture element area 2 by utilizing a bonding resin 10.
In case that the solid-state image sensor mentioned above is applied to an image sensor of a color camera, when an incident light .dotthalfcircle.A shown in FIG. 2 of spot and high brightness is irradiated through a lens of the color camera to the solid-state image sensor, many carriers are generated in the photo element area 2 which includes the photo sensors 12. In case that the quantity of the generated signal carriers exceeds the signal charge transfer capacity, an excessive signal carrier .circle.B is diffused to the photo element area 2 or a signal transferring area, both areas being within the circumference of the irradiated part, and accordingly the blooming occurs.
On the other hand, for producing a monoplate solid-state color image sensor, a mosaic shaped color filter or a striped color filter has to be disposed in front of the photo diodes.
FIG. 3 is a sectional view taken on line X--X' of a solid-state color image sensor of FIG. 1 with a detailed illustration for a color filter. In FIG. 3, every part corresponding to FIG. 2 is designated with the same reference numerals and marks. A color filter 11 comprises a glass plate 13 with a color filter layer 14 formed under the lower face thereof. The color filter 11 is bonded above the picture element area 2 by utilizing a bonding resin 10. In the case of another conventional device, the color filter layer 14 can be formed directly on an upper face of a photoconductive film 8 without utilizing a bonding resin.
But in the above-mentioned solid-state image sensor, the spectral transmittance characteristic of a color filter appears as shown in FIG. 4, in an example of a three color (R, G, B) system.
FIG. 4 is a graph of the characteristic curves showing spectral transmittance characteristics of a general color filter. In the graph, ordinate is graduated by optical transmittance (%), and the abscissa is graduated by wave-length (nm) of irradiated light. Referring to FIG. 4, it is recognized that only the hatched part of the irradiated light can reach the photo sensor and be effectively transduced to a photo current. That is to say, in the color image sensing system, the sensitivity of the system considerably decreases as a necessary consequence, in comparison with the monochrome image sensing system. Therefore, for developing the sensitivity of the color image sensing system, it is necessary to improve the optical sensitivity of the photo sensor, or to increase the signal amplification factor by improving the charge transferring efficiency, or to reduce the noise in a transferring area (not shown) and/or the driving area (not shown). But in the conventional semiconductor producing process (i.e. the process for producing a solid-state image sensor), it is difficult to realize enough noise reduction in the transferring area or the driving area.
On the other hand, for improving the blue light sensitivity, it is proposed that the diffusion depth of the photo diodes should be made shallow in the photo sensor with silicon photo diodes on a semiconductor substrate. And further, as an alternative, it is proposed that ZnSe--Zn.sub.1-x Cd.sub.x Te or an amorphous silicon may be applied as the photoconductive film 8 instead of the silicon photo diodes.
But even by using the method mentioned above, only such a spectral sensitivity characteristics as shown in FIG. 5 is attainable, which is not satisfactory.
FIG. 5 is a graph of characteristic curves showing the spectral sensitivity characteristics of conventional photo sensors. In the graph, the ordinate is graduated by relative sensitivity (%), and abscissa is graduated by wave-length (nm) of irradiated light. A characteristic curve of the solid line shows the spectral sensitivity characteristic of a conventional solid-state image sensor with the ZnSe--Zn.sub.1-x Cd.sub.x Te film. The characteristic curve of the dotted line shows a spectral sensitivity characteristic of another conventional solid-state image sensor performing photoelectric transducing only by silicon photo diodes in a semiconductor substrate without utilizing a photoconductive film.
As above-mentioned, even utilizing the ZnSe--Zn.sub.1-x Cd.sub.x Te film, only a little photocurrent is practically obtainable because of the decrease of the constituent of blue light (wave-length is 400-480 (nm)), and as a result, the sensitivity is generally low.